The lithium secondary battery has recently been widely used for example, as an electric source for driving small-sized electronics. The lithium secondary battery has a basic structure comprising a positive electrode, a negative electrode and a non-aqueous electrolytic solution. The positive electrode preferably comprises a complex oxide of lithium such as LiCoO2, and the negative electrode preferably comprises a carbon material or metallic lithium. A carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) has been advantageously used in the non-aqueous electrolytic solution for the lithium secondary battery.
The recent secondary battery requires a further improvement on battery performance such as cycle characteristics of the battery and electric capacity.
In a lithium secondary battery, LiCoO2, LiMn2O4 or LiNiO2 is used as a positive electrode material. A process of charging the battery causes a local oxidation and decomposition reaction of a part of a solvent contained in a non-aqueous electrolytic solution. The decomposition product inhibits an ordinary electrochemical reaction of the battery to lower battery performance. The reason is considered that a solvent is electrochemically oxidized along the interface between the positive electrode material and the non-aqueous electrolytic solution.
In a lithium secondary battery, a highly crystallized carbon material such as natural or artificial graphite is used as a negative electrode material. A process of charging the battery causes a reduction and decomposition reaction of a part of solvents contained in a non-aqueous electrolytic solution on a surface of the negative electrode. Ethylene carbonate (EC) is widely used as the non-aqueous electrolytic solvent. Ethylene carbonate may partially be reduced and decomposed to lower battery performance while repeating charge and discharge.
U.S. Patent Application Publication No. 2002/0110735 discloses an invention of adding a pentafluorobenzene compound having an electron-donating group such as 2,3,4,5,6-pentafluoroanisole to a non-aqueous electrolytic solution to improve battery performance of the lithium secondary battery. The obtained capacity retention of a coin-shaped cell after 200 cycles is 80%, which is not necessarily satisfactory.
Japanese Patent Provisional Publication No. 7-302614 discloses a chemical method for protecting a battery from excessive charge. The publication describes that a non-aqueous electrolytic solution can contain 2,3,4,5,6-pentafluoroanisole as a redox agent according to the method. However, the publication is silent with respect to cycle characteristics.
Japanese Patent Provisional Publication No. 11-329490 describes that a non-aqueous electrolytic solution can contain a specific pentafluorobenzene derivative to improve battery performance of the lithium secondary battery, such as cycle characteristics, electric capacity, and storage characteristics.
U.S. Pat. No. 5,626,981 discloses that a non-aqueous electrolytic solution can contain vinylene carbonate to improve storage stability of the lithium secondary battery.
U.S. Pat. No. 6,033,809 discloses that a non-aqueous electrolytic solution can contain 1,3-propanesultone and/or 1,4-butanesultone to improve battery performance such as cycle characteristics, electric capacity, and storage characteristics as well as performance at low temperature.